(1) Field of the Invention
The present invention relates to dual mode radio frequency (RF) amplifiers and, in particular, to a unique matching network operable therewith.
(2) Description of the Prior Art
The invention is particularly useful in mobile terminals, such as. personal communication assistants, pagers, head sets, wireless modems, analog and digital cellular telephones, and the like. Since many of these devices are battery-powered, amplifier efficiency is preferably maximized to extend battery life. When amplifiers are designed for their highest efficiency in converting DC energy into RF energy, parasitic losses are minimized, bandwidths are reduced to their bare minimum, harmonics are terminated, and high-Q matching networks are employed. Unfortunately, these design goals are counter to current approaches used to implement wide-band amplifiers or dual-mode amplifiers capable of operating at two different frequencies.
Most RF power amplifiers are designed to operate over a single band of frequencies. In many cases, the bandwidth is less than 5% of the center frequency. If simultaneous coverage over a second band or an extension of the original frequency band is desired, specialized design techniques are required. Wide-band amplifiers are typically created by 1) providing feedback around the power amplifiers, 2) reducing the Q of the matching network, 3) increasing the complexity of the matching network, or 4) employing distributed techniques. Each of these approaches reduces the efficiency of the amplifier by adding losses into the system as part of the trade-off for increased bandwidth. Reducing amplifier efficiency is undesirable, especially in wireless communication applications using battery-powered mobile terminals.
If coverage is desired for an additional frequency band, a dual-band amplifier is typically created. The most straight-forward approach is to simply use two amplifiers and switch between the amplifiers to select a desired band for transmission. If a single amplifier configuration is desired, then matching networks that provide the proper impedance transformation for both frequency bands are required. These matching networks are implemented in a number of ways. The use of series and parallel resonant elements (typically inductors and capacitors) in the matching networks are selected such that, at one frequency band, the combination appears inductive and at the other band it appears capacitive. Clever combinations of series and shunt element pairs may allow a creation of networks that deliver the desired properties over multiple frequency bands. These approaches typically result in limited bandwidth for each band and difficulty in tuning and maintaining performance over each of the bands.
If the amplifier is designed to operate in one band at a time, one or more switches are typically used to add or remove elements to or from the matching networks for the appropriate band. This works well and has been employed in many applications. The drawbacks are the additional energy needed to operate the switches and the losses the switches add to the networks. If a change in the amplifier mode is desired, such as changing linearity, efficiency, or power, then the load of the amplifier may be switched by adding or removing elements from the matching network. Examples of these techniques are disclosed in U.S. Pat. Nos. 5,438,684 and 5,673,287 which are assigned to Motorola, Inc. Again, the switching techniques reduce system efficiency, which results in decreased battery life. There is a need for an improved and efficient, dual mode matching network and amplification technique that does not require switches or wide-band tuning of the network.
The present invention provides a novel solution to the problems discussed above. In particular, the present invention provides a dual mode amplifier capable of operating in a common (even) mode for one frequency band and a differential (odd) mode for a second frequency band. In the common mode, the amplifier provides two identical signals to a matching network, and in the differential mode, the amplifier provides two signals that are 180xc2x0 out-of-phase from one another to the matching network. The matching network is configured to maintain the same input and output impedance regardless of whether the amplifier is operating in the common or differential mode. Furthermore, the matching network is preferably configured to terminate second harmonics for each frequency band without affecting the fundamental tone for the other bands, even if the bands are an octave apart. Since the matching network operates on two signals, either common or differential signals, a power combining network is typically required to combine the two signals into a single signal for transmission.